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FAILURE ANALYSISA Guide to Analyzing Axial Piston

Pump Failures

Oilgear CompanyMilwaukee, USA

80-776, Rev. 4/05

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Typical Failure Modes

• Contamination• Fluid Issue• Over Pressurization• Improper Inlet Condition• Case To Inlet Differential• Miscellaneous

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CONTAMINATION10 Micron,Beta 10 of 4 or Better

ISO Contamination Grade of 21/19/16

PISTON SEIZED IN BORE, PULLS SHOE OFF

PISTONS SHOW FINE SCRATCHES,DULL FINISH

EXCESSIVE WEAR ON SWASHBLOCK FACE, SHOE FACE AND VALVE PLATE FACE

EXCESSIVE WEAR ON SADDLE BEARINGS

HYDRO-DYNAMIC BEARING WORN

CONTROL UNSTABLE: PISTON STICKING, COMPENSATOR SPOOL STUCK OR WORN, ORIFICE IN CONTROL PISTON PLUGGED

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Note Metal Transfer

PISTON THAT

SEIZED IN BORE

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VERTICAL SCRATCHES:A Clear Indication of Contamination

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DULL AND SCRATCHY

FINISH

Normal Appearance: A Mirror Finish

Abnormal Appearance: A Steel Wool Look

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SHOE WEARPLATE

SHOWINGSIGNS OF WEAR

DUE TO DIRT

Scratchy Uneven Wear

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PISTON SEIZED IN BORE, PULLS PISTON SHOESHOE FACES AND OR VALVE PLATE FACE SMEAREDBALL WORN THROUGH SHOE RETAINERSHAFT SEAL LEAKSCAVITATION, AIR ENTRAINMENTEXCESSIVE SADDLE BEARING WEAR

Fluid IssueViscosity too Low, Operating Temperature too

High, Not a Hydraulic Fluid, Fluid Breaking Down

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VALVE PLATE RUN

ON A BAD FLUID

Note the Smeared Running Surface

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WORN FULCRUM

BALL

Typical Failure on Low Lubricity Fluid

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WORN SADDLE

BEARING

Note the Material has Delaminated

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OVER PRESSURIZATION/SPIKESSpike Relief Always Recommended

Excessive swashblock face, cylinder to valve plate wear

Pistons broken at necks

Broken shaft where cylinder rides

Broken tail shaft (dual)

Control pin broken

Cylinder cracked between kidneys

Control O-ring, gasket failure (PVW)

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Cracks Would Appear Between Kidneys

EXCESSIVEWEAR ONCYLINDER

FACE

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TORSIONAL FATIGUE:Note Cone Shaped Failure

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BENDING FATIGUE FAILURE

Note Straight Break

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IMPROPER INLET CONDITIONSSuction Strainer not Recommended

CAVITATION ON VALVE PLATE FACE

NOISE (MARBLE SOUND)

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CAVITATION ONCOMPRESSION

BRIDGE

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CASE TO INLET DIFFERENTIALIn General Differential Cannot Exceed 10 PSI

SHOE EDGES ROUNDED

SHOES LOOSE ON BALLS

SWASHBLOCK WEAR, HALF MOON

SHAPE

EXCESSIVE WEAR ON BACK OF SHOE

FLANGES

SEAL RETAINER BENT

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WHAT DOES CASE PRESSURE DO ?

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TROUBLE !!!!!

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STAGES OFDAMAGEDUE TO

SHOE LIFT

3rd Stage2nd Stage1st Stage

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THE FINALSTAGE

OFSHOE LIFT

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HALF MOON SHAPED MARKS

A Sure Sign of Shoe Lift

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ROLLED SHOES: THE TELL TALE OF SHOE LIFT

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WEAR ON SHOE RETAINER FROM SHOE FLANGES

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BENT SEAL RETAINER

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MISCELLANEOUSINPUT SHAFT BROKEN8MISALIGNMENT8TORQUE REVERSAL

CONTROL INSTABILITY8AIR8INCREASE CONTROL PISTON ORIFICE

PUMP VIBRATES8MISALIGNMENT8COUPLING HALF'S TOUCHING

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HELPFUL HINTS FOR ANALYZING BASKET CASES

The last piece to fail will have the least amount of damage

Try to piece together the broken parts. This may seem like a waste of time but many times you will observe things which can help you determine the original cause of failure.

Get the history of events:

8What recently changed ?

8What was going on just prior to failure ?

8How long was pump run ?

8Talk to the operators

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KEYS TO FAILURE ANALYSIS

Don’t go into a customers problem with a preconceived idea about the cause

Don’t assume anything, verify everything

Don’t overlook the obvious